Moreover, the containers bulge--unless they are filled completely with liquid--in balloon-like fashion under the influence of external heat. This also happens in the case of sheet metal canisters, but not to the same extent. The plastic canisters then fit no longer into standard holders or can no longer be taken out of these standard holders. If the containers are fuel tanks in motor vehicles, they do not have to be removed. Nevertheless, the designer must make provisions that the tanks are able to change their shape in the manner required.
In the cast of stationary tanks, for example, in basements or buried in the ground, the changes do not play a major part since it is in this case possible to install vents which, in contrast to the tanks in motor vehicles, never become clogged. Here, however, the hydrostatic pressure on the side walls does play a part.
Another fact which plays a part in the case of 20- and 30-l canisters is that they are, when dropped from a height, briefly deformed into a pear-like shape and that the walls must be capable of following this deformation without force peaks, because otherwise additional forces would act on the walls.
Generally speaking, plastic canisters would be superior to metal canisters for various reasons because they are, for example, considerably lighter in weight than metal canisters, do not have to be painted, no rust flakes off on the inside as time goes on, they do not rattle, and so on.
However, the efforts to stabilize the side walls have thus far failed to lead to a practical solution.
For example, it has not been possible to produce a practically useful brace according to FIG. 1 of the Brit. Pat. No. 1,007,563, because the deep-drawing ratio is much too large and the walls of the braces are then too thin. This applies not only to the walls of the braces but above all to the angle regions. However, a canister is only as strong as its weakest point. Moreover, such braces represent very troublesome structural elements when it is a question of improving the drop resistance of canisters because they try not to yield either to tension or the compression. Finally, these braces take away a very considerable volume. However, for example, a 20-l canister has a geometry specified by standards within which 20 liters must be accommodated. This cannot be done if part of the volume is sacrificed in other ways.
In the case of the device according to U.S. Pat. No. 3,552,599, an attempt has been made to eliminate the large deep-drawing ratio by realizing the braces by two flat cones. This, however, leads to a large loss of volume, particularly when, for example, in the case of larger containers, four such braces must be provided.
Here again--as in the other state of the art--the brace must be formed of blown plastic material which, because of the limited blowing pressures, is known to be flabby. Although it may do for a canister wall, it performs the bracing function very poorly. For example, at the point 76 in FIG. 5 of this U.S. patent, this material offers practically no resistance to notch cracks.